This invention relates to a high breakdown voltage semiconductor device, particularly to a high breakdown voltage semiconductor device employing an SOI (Silicon On Insulator) substrate.
So-called power IC, wherein a high breakdown voltage semiconductor device and a peripheral circuit including a driving circuit of the high breakdown voltage semiconductor device and a protection circuit are integrated on one substrate is expected to be used in the field of power electronics, as a general purpose inverter, the IC used in a car, and the driver for a display. The power IC uses an SOL substrate since it can easily isolate the elements in the IC, and thus is convenient.
FIG. 1 is a plan view showing a lateral type high breakdown voltage diode formed on a conventional SOI substrate. FIG. 2 is a sectional view of the diode taken along a line II--II. In these drawings, 81 denotes the first silicon substrate above which an n-type second silicon substrate 83 is formed to hold an SiO.sub.2 film 82 therebetween.
The first silicon substrate 81, the SiO.sub.2 film 82, and the n-type second silicon substrate 83 constitute the SOI substrate. The SOI substrate is formed in accordance with a method such as the bonding method or the SIMOX (Separation by IMplanted OXygen) method. The n-type second silicon substrate 83 (hereinafter referred to as an n type drift layer) has a high impurity p-type anode layer 84 and an n-type cathode layer 85, selectively formed on the surface thereof.
The n-type second silicon substrate 83 has an interlayer insulating film 86 thereon. An anode electrode 87 and a cathode electrode 88 contact with the p-type anode layer 84 and the n-type cathode layer 85 respectively, via contact holes formed in the interlayer insulating film 86.
In the space between the p-type anode layer 84 and the n-type cathode layer 85 on the surface of a n-type drift layer 83, an LOCOS (LOCal Oxidation of Silicon) film 89 is selectively formed. By adding the LOCOS film 89 in this manner, an insulating film formed under the interconnection wire 88a of the cathode electrode 88 is formed thick by the thickness of the LOCOS film 89 to increase the breakdown voltage of the device.
Such a lateral type high breakdown voltage diode, however, has the following problem:
The n-type drift layer 83 needs to be formed thick to ensure the necessary breakdown voltage, and the impurity concentration in the n-type drift layer 83 is set at a low level, in general. In such a structure, the ON resistance in this device is high.
In such a device, a potential inclination is generated on the surface of the n-type drift layer 83 (i.e., the element surface) in applying reverse-bias voltage, in which the potential on the side near the p-type anode layer 84 becomes lower than that on the side near the n-type cathode layer 85. With such a structure, the potential inclination as mentioned above is increased when the device is decreased in a chip size, and the breakdown voltage on the surface is deteriorated, as a result.
In consideration of the above, a high breakdown voltage semiconductor device which can prevent the deterioration of the breakdown voltage and the increase of the ON resistance, and prevent the deterioration of the breakdown voltage of the device without increasing the chip size of the element has been waited for.